Semiconductor wafer processing method and apparatus with heat and gas flow control

ABSTRACT

A semiconductor wafer processing apparatus is provided with a susceptor for supporting a wafer for CVD of films such as blanket or selective deposition of tungsten or titanium nitride, and degassing and annealing processes. Preferably, a downwardly facing showerhead directs a gas mixture from a cooled mixing chamber onto an upwardly facing wafer on the susceptor. Smooth interior reactor surfaces include baffles and a susceptor lip and wall shaped to minimize turbulence. Inert gases flow to minimize turbulence by filling gaps in susceptor structure, prevent contamination of moving parts, conduct heat between the susceptor and the wafer, and vacuum clamp the wafer to the susceptor. A susceptor lip surrounds the wafer and is removable for cleaning, to accommodate different size wafers, and allows change of lip materials to for different processes, such as, one which will resist deposits during selective CVD, or one which scavenges unspent gases in blanket CVD. The lip smooths gas flow, reduces thermal gradients at the wafer edge. The susceptor design reduces heat flow from the susceptor to other reactor parts by conduction or radiation.

This application is related to commonly assigned and copending U.S.application Ser. Nos. 07/898,826 entitled ROTATING SUSCEPTORSEMICONDUCTOR WAFER PROCESSING CLUSTER TOOL MODULE USEFUL FOR TUNGSTENCVD, and 07/898,560 entitled SEMICONDUCTOR WAFER PROCESSING CVD REACTORCLEANING METHOD AND APPARATUS, both filed on Jul. 15, 1991 by theinventors hereto.

The present invention relates to chemical vapor deposition (CVD)reactors for the processing of semiconductor wafers, and particularly toCVD reactor modules for semiconductor wafer processing cluster tools.More particularly, the present invention is applicable to the CVD ofcoating materials such as tungsten, titanium nitride and other metal anddielectric substances onto silicon semiconductor wafers, and to othersilicon processes in which mass transport is a present but notcontrolling characteristic of the process.

BACKGROUND OF THE INVENTION

In the manufacture of semiconductor wafers and of other similarlymanufactured articles, sequences of processes including coating,etching, heat treating and patterning are sequentially employed. Most ofthese processes involve the chemical or physical addition or removal ofmaterial to or from a surface of a substrate, usually transported as avapor.

Certain coating processes in such sequences are performed by chemicalvapor deposition (CVD). CVD is preferred, for example, in applying filmsto the differently facing surfaces of holes through underlying layers,as, for example, to apply conductive films for the purpose of makinginterconnections across insulating layers and the like.

The ultimate result of CVD processes for filling holes or vias, and forforming interconnections between layers on semiconductor wafers, isfrequently the selective deposition of the film, that is, formation of apermanent film on only selected portions of the wafer surfaces. Directselective application by CVD of such coatings is often unreliable,unsuccessful, or slow, and thus undesirable on a commercial scale, whererapid throughput and efficient use of expensive machinery is important.Therefore, selective end product films are often applied in blanketfashion and then etched back from the areas where permanent film isundesired.

Blanket CVD of materials, such as tungsten, followed by an etching backof the deposited material, requires a high degree of uniformity in theblanket film, particularly on the areas of a substrate from which thematerial is to be etched. If the coating is irregular in the etch-backareas, the etching process may selectively damage the underlying layersin regions of the wafer where the blanket film to be etched is thin, ormay result in regions where residual film remains. CVD reactors of theprior art have coated substrates with limited uniformity, or at limitedspeed. Accordingly, more uniform application of the films and higherspeed CVD reactors, particularly for blanket coating applications ofmaterials such as tungsten, are required.

To uniformly apply films such as tungsten by CVD to semiconductorwafers, it is desirable to ensure a uniform supply of reactant gasesacross the surfaces of the wafers, and to uniformly remove spent gasesand reaction byproducts from the surfaces being coated. In this respect,prior art CVD reactors perform with limited success. Similarly, in otherprocesses such as physical and chemical etching and heat treatingprocesses, including preheating and annealing processes, prior artsystems have been inadequate in uniformly bringing vapors into contactwith, and removing them from, the surface being processed. Accordingly,there is a need to more efficiently and more uniformly supply and removereaction and other gases to and from the surfaces of wafers beingprocessed, and particularly those being coated by CVD processes.

Efficient commercial production of semiconductor wafers requires thatthe processing equipment function as continuously as possible. Whendeposits form on interior components of processing chambers, such asthose of CVD reactors, they become ineffective and their use must besuspended for cleaning. Many reactors of the prior art require cleaningat an undesirable frequency, or are too difficult and too slow to clean,thus resulting in excessive reactor downtime. Accordingly, there is acontinuing need for processing chambers such as those of CVD reactorsthat require less frequent cleaning of components, that reduce unwanteddeposition on components, and that can be cleaned more rapidly.

In the chambers of CVD reactors and other wafer processors of the priorart, turbulence in the flow of reaction gases has inhibited theefficiency and uniformity of the coating process and has aggravated thedeposition and migration of contaminants within the reaction chamber.Accordingly, there is a need for improved gas flow, and reduced gas flowturbulence, within such chambers.

CVD processes such as those for the application of tungsten coatings tosemiconductor wafers are typically performed in cold wall reactors,where the wafers to be coated are heated to a reaction temperature on asusceptor while other surfaces of the reactor are maintained atsubreaction temperatures to prevent the deposition of films thereon. Fortungsten CVD, for example, reactor walls are often cooled, often toabout room temperature. Alternatively, for titanium nitride (TiN) CVD,the walls may be heated above room temperature, but to a temperaturebelow that of the substrate being treated. In such cases, there is aneed in the designs of such wafer processing devices that havecomponents that are maintained at different temperatures to prevent heatfrom flowing between the wafer or susceptor and other components of theapparatus.

In tungsten CVD processes, tungsten hexafluoride gas (WF₆) is commonlyemployed. This WF₆ gas is costly, as are the gases employed in manyother wafer treating processes. When the gas utilization efficiency islow, as is the case of many reactors of the prior art, the cost of thegas can be high. With many tungsten CVD reactors, the utilizationefficiency of WF₆ is below twenty percent, and the cost of the WF₆ oftenexceeds thirty percent of the entire cost of the performance of theprocess for application of the tungsten film. Accordingly, CVD reactorsthat are more efficient in the consumption of reactant gases such as WF₆are required.

CVD processes may be divided into two categories, those that are masstransport controlled and those that are surface condition or temperaturecontrolled. Mass transport controlled processes are typically thoseinvolving the CVD of group III-V materials onto substrates such asgallium arsenide wafers or for the epitaxial growth of silicon. Suchprocesses are controlled by the transport of gases to and from the wafersurfaces and have been used by moving the wafers, typically mounted inpluralities on rotating or otherwise moving susceptors that cause thesubstrates to orbit about an axis in a flowing gas, or otherwiseemploying techniques to enhance and control the gas flow across thewafers. Typically, the mass transport controlled CVD processes will befound on an Arrhenius plot, that is a plot of the log of the depositionrate versus the reciprocal of the temperature, above the knee in thecurve.

Wafer temperature or surface condition controlled CVD processes aretypically found below the knee of the Arrhenius plot curve. These arebrought about by lower temperatures, and usually at lower pressures offrom 1 to 100 Torr. Generally, such processes are not regarded in theprior art as amenable to enhancement by wafer movement, except toachieve temperature or reaction uniformity, which is promoted with lowspeed movement.

SUMMARY OF THE INVENTION

It has been a primary objective of the present invention to provide anefficient and productive apparatus for the thin film processing ofarticles such as semiconductor wafers. It is a more particular objectiveof the present invention to provide such an apparatus primarily usefulfor the chemical vapor deposition of films onto semiconductor wafers,as, for example, the blanket or selective deposition of, for example,tungsten, titanium nitride and similarly materials deposited by CVDprocesses onto silicon semiconductor wafers.

It has been a further objective of the present invention to provide insuch a processing apparatus, such as a CVD apparatus for uniformlyapplying film to semiconductor wafers, such as for applying blanketfilms of tungsten, other metal and dielectric material, by CVDprocesses, and other silicon processes that are primarily surfacetemperature controlled and dependent, that is effective in enhancing therate and quality of the effective in enhancing the rate and quality ofthe wafer coating or other processed surface, whether planar or, moreimportantly non-planar or patterned.

It has been another objective of the present invention to provide asealed chamber processing apparatus, such as a cold wall CVD reactor,having minimal heat flow from the heated wafer or susceptor to othercomponents of the apparatus that should remain cooler.

It has been a further objective of the present invention to provide aprocessing apparatus requiring less downtime for cleaning, having aresistance to the buildup of unwanted deposits within and to thepropagation of contaminants therethrough, and that is efficientlycleaned.

It is a particular objective of the present invention to enhance theuniformity of the application of coating in a CVD processing apparatus,to maintain clean internal surfaces, and to thermally isolate the heatedwafer or susceptor, by reducing the turbulent flow of gases within thereactor.

Further objectives of the present invention include providing for inertgas flow within the reactor to facilitate the holding of the wafer tothe susceptor, to enhance the conduction of heat between the wafer andthe susceptor, to protect internal components of the reactor fromundesired deposits and contamination, and to assist the non-turbulentflow of reactant gases through the reactor.

It is a further objective of the present invention to provide a CVDreactor which will easily accommodate wafers of differing sizes forcoating and accommodate different coating processes.

It has been a particular objective of the present invention to provide aprocessor and module for a wafer processing cluster tool or stand-aloneprocessor utilizing a single wafer rotating susceptor, and particularlyone for the chemical vapor deposition of films, such as blanket films,of materials such as tungsten, titanium nitride, and other such filmsamenable to such processes, and alternatively for the selectivedeposition of such materials.

According to the principles of the present invention, there is provideda CVD processing apparatus with a reactor having a single wafer rotatingsusceptor on which a wafer is maintained at a processing temperature,and having a reactor wall maintained at a different temperature. Inreactors for the application of films such as tungsten, the walls arecooled to approximately room temperature while in those for theapplication of titanium nitride films, the walls are heated to aboveroom temperature but to below the optimum processing temperature of thesusceptor.

The preferred embodiment of the present invention provides a CVD modulefor a wafer processing cluster tool having a rotating wafer holdingsusceptor that rotates on a vertical axis, is preferably upwardlyfacing, and has a flow of reactant gas directed from a showerhead,preferably downwardly, toward and perpendicular to the wafer, with thesusceptor rotating sufficiently fast to cause a thin boundary layer toform above the wafer surface, across which the gases that interact withthe wafer surface diffuse. In the CVD reactor, reactant gases flowradially outwardly from a stagnation point at the wafer center on theaxis of rotation.

In the preferred embodiment of the invention, gases are caused to flowwith minimum turbulence from a downwardly facing showerhead at the topof the chamber, downwardly against the upwardly facing wafer surface,radially outwardly across the wafer surface, over a wafer encirclingring or lip, downwardly along the susceptor side-wall, through annularopenings defined by baffles, and then out a single vacuum exhaust portin the end or bottom of the chamber opposite the showerhead. In CVDapplications, plasma cleaning electrodes are provided and are combinedwith structure shaped to facilitate non-turbulent gas flow. The walls ofthe susceptor have finishes and cross-sections that retard the flow ofheat from the heated components to the cooled components of the reactor.

In one alternative embodiment of the invention, an inert gas isintroduced at points around the wafer, the wafer support and thesusceptor rotating structure to inhibit contamination thereof byparticles and reactant gas and to facilitate the smooth flow of thegases through the chamber across junctures of the susceptor components.In other embodiments, inert gas is employed for retention of the wafer,by relative vacuum, to the susceptor, and to enhance heat conductionbetween the susceptor and the wafer. In embodiments where inert gas isintroduced around the rim of the wafer and for vacuum clamping of thewafer to the susceptor, the inert gases are introduced from separatesupplies, with the rim gas introduced at or above the processing chamberpressure and the vacuum clamping gas introduced at a lower pressure.

In accordance with the preferred and illustrated embodiment of thepresent invention, there is provided a CVD reactor having an upwardlyfacing rotary susceptor spaced below a horizontally disposed, downwardlydirected reactant gas distributing showerhead that separates a gasmixing chamber from a reaction chamber that encloses the susceptor. Themixing chamber, located at the top of the reaction chamber, ismaintained at a relatively low, sub-reaction temperature, along with thewalls of the reaction chamber. In tungsten deposition applications, themixing chamber and the reaction chamber walls are cooled toapproximately room temperature, either with ethylene glycol, water orsome other suitable fluid, while in titanium nitride depositionapplications, these are heated to a temperature between room temperatureand the reaction temperature of the susceptor.

During a deposition reaction, the susceptor is rotated. For 150millimeter wafers in a tungsten deposition process, with reactionpressures at about 50 Torr, the susceptor is rotated at least at 200RPM, preferably at not more than 2000 RPM, and most preferably in therange of from 500 to 1500 RPM. The rotation results in a stagnationpoint at the center of the wafer and minimizes the thickness of theboundary layer immediately above the surface of the wafer, enabling theprocess gas to reach the wafer faster and the by-products from theprocess to escape from the upper surface of the wafer. As such, thesefeatures present advantages not only in deposition processes such asCVD, but in etching processes and other processes where gases mustefficiently be brought into contact with the wafer surface orefficiently removed from the surface, such as annealing and degassingprocesses and other heat treating processes.

In the preferred and illustrated embodiment, the susceptor is heated toapproximately 400°-550° C., preferably 450° C., and heat from the heatedsusceptor is prevented from significantly heating the rotary shaft onwhich the susceptor is supported by its mounting and its fabrication.Highly reflective surfaces on all elements inside the rotating susceptorminimize heat transfer between the heated wafer support of the susceptorand the drive assembly. In addition, a dull surface finish is providedon the exterior of the rotating susceptor to maximize radiation of heataway from the susceptor toward the chamber walls where the walls arecooled, and to minimize absorption of heat from the chamber walls wherethe chamber walls are heated. Extremely thin susceptor walls furtherminimize heat transfer between the heated wafer support and the driveassembly.

The chamber walls are also thermally isolated from the drive assembly. Asusceptor mounting disc connects an annular flange on the susceptor basewith the top of the susceptor drive shaft, and is provided withprojecting support structure to present a minimum contact surface to actas a thermal block to further reduce heat transfer between the heatedwafer support and the drive assembly.

The reactant gas is caused to flow from the showerhead with minimumturbulence, downward to a single exhaust outlet in the bottom of thereaction chamber. Multiple baffles at the bottom region of the chamberencircle the susceptor shaft and provide annular gas flow openingsaround the shaft to present progressively decreasing cross-sectionalarea to the gas flow, thus providing a pressure gradient thatfacilitates exhausting gas uniformly through the single port in thechamber floor without creating turbulence inside the chamber. Theexterior shape or envelope of the rotating susceptor is smoothlycontoured to further minimize turbulence. An annular lip is providedaround the wafer on the upper surface of the heated wafer support, andis closely spaced to the circular edge of the wafer and flush with theupper surface thereof to further minimize turbulence and to alsoeliminate radial thermal gradients in the wafer in the edge regionthereof. The wafer-encircling lip is a separate annular element whichcan be readily removed and substituted with a different one having adifferent internal diameter to accommodate wafers of different sizes.Rounded corners on the upper circular edge of the annular lip elementfurther minimize turbulence. This lip element has a substantial upwardlyfacing annular surface to serve as a scavenger for unused tungstenhexafluoride gas or other reactant gas, thereby minimizing the amount ofreactant gas which has to be scrubbed from the exhaust.

In this preferred embodiment, a pair of annular electrodes are provided,one at the top of the chamber and one at the bottom of the chamber, forplasma cleaning of the reactor. Each of these electrodes is providedwith openings for injection into the chamber of NF₃ gas, in the case oftungsten deposition, or some other cleaning gas appropriate to theprocess. The openings are disposed in circular arrays in the upper andlower electrodes to facilitate plasma cleaning of the interiorcomponents of the chamber. The upper electrode has a conical innersurface with an angulation from the diameter of the showerhead towardthe reaction chamber wall, which also contributes to the minimization ofturbulence. The lower electrode is incorporated into the uppermost oneof the baffles.

Further in accordance with one alternative embodiment of the invention,nitrogen gas passages above and below a vacuum passage, all of whichpassages encircle the drive-shaft in the base of the chamber, reducereactant gas and particulate contamination of the bearings and othershaft supporting and moving structure. This feature is preferred whereit is desirable to extend the service life of the bearing and increasethe time required between servicing.

In embodiments where vacuum holding of the wafer to the susceptor isemployed, helium leakage paths are provided around wafer lifting pins onthe susceptor surface, which facilitates vacuum gripping of the waferwhen the pressure in the rotating susceptor interior is maintained belowthe pressure of the CVD chamber. Further, helium gas below the waferbetween the back of the wafer and the upper surface of the heated wafersupport, which, unless the pressure thereof is too low, will provideheat transfer between the back side of the wafer and the wafer supportthrough gas phase thermal conduction.

The preferred embodiment of the invention is most advantageous forblanket CVD of tungsten, performed at pressures of from 10-100 Torr andat wafer temperatures at from 425°-525° C. The process is preferablyperformed with a nucleation step in which WF₆ is reduced with silane,followed by a deposition step in which WF₆ is reduced with hydrogen.

In an alternative embodiment of the invention, the reactor can beadvantageously used for the selective deposition of tungsten, intendedto coat contacts and fill vias. With this embodiment, the susceptorsurfaces contacting the wafer, particularly the lip surrounding the edgeof the wafer and the upper support surface on which the wafer rests, aswell as the exposed screws and devices fastening them and the ring sealscontacting them, are formed of material on which tungsten either willnot nucleate or will nucleate only in an unacceptably long nucleationtime. Such materials may include aluminum oxide, boron nitride,polyimide and some forms of quartz. Furthermore, the lip ring and uppersupport surface of the susceptor are removable and replaceable, thesusceptor can be converted between non-selective applications.

In this selective tungsten deposition application, the process ispreferably performed at pressures from 0.1-10.0 Torr and at temperaturesfrom 250°-400° C. At these lower pressures, the wafer may be held on thesusceptor with electrostatic clamping, rather than a vacuum behind thewafer.

The apparatus of the present invention achieves the objectives set forthabove and overcomes problems of the prior art. Used with blankettungsten deposition processes, in excess of 50% WF₆ consumption may berealized, and deposition rates several times higher than conventionallyachieved can be obtained.

The present invention is particularly advantageous in enhancing thespeed of application, quality and uniformity of CVD applied films ontosilicon wafers by wafer temperature controlled processes, and to othertemperature controlled silicon processing methods such as annealing.Many features of the present invention are advantageous in the CVD ofblanket tungsten, selective tungsten and titanium nitride onto siliconsemiconductor wafers, and to the CVD of other materials such as tungstensilicide, tantalum oxide, aluminum and copper, as well as oxides such asSiO₂.

Many features of the present invention are useful with processingdevices that do not include the rotating susceptor. The plasma cleaningfeatures of the present invention provided advantages when used with thesemiconductor wafer processing applications discussed herein, and otherapplications where deposits and contaminants tend to form. In addition,the features that confine the heat to the susceptor, and those thatenhance the flow of gases in the reactor in a smooth and non-turbulentmanner, have broad utility in semiconductor wafer processing.

These and other objectives and advantages of the present invention willbe more readily apparent from the following detailed description of thedrawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevational view of a CVD module for a wafer processingcluster tool embodying principles of the present invention.

FIG. 2 is a cross-sectional view of a CVD reactor of the module of FIG.1.

FIG. 3 is a cross-sectional view of the lower part of the reactor ofFIG. 2 illustrating the susceptor rotation and wafer lifting portion.

FIG. 3A is a cross-sectional view taken along line 3A--3A of FIG. 3.

FIG. 4 is a cross-sectional view of the upper part of the reactor ofFIG. 2 illustrating the processing chamber portion.

FIG. 4A is a cross-sectional view taken along line 4A--4A of FIG. 4.

FIG. 4B is a cross-sectional view taken along line 4B--4B of FIG. 4.

FIG. 4C is a cross-sectional view taken along line 4C--4C of FIG. 4.

FIG. 5 is an enlarged cross-sectional view of a part of the chamber ofFIG. 4 illustrating the structure in association with the susceptordrive shaft in the vicinity of the base of the housing of the reactionchamber in one alternative embodiment.

FIG. 6 is an enlarged cross-sectional view of the susceptor within thereaction chamber of FIG. 4.

FIG. 6A is a cross-sectional view taken along line 6A--6A of FIG. 6.

FIG. 6B is an enlarged cross-sectional view, similar to FIG. 6, of thesusceptor of an alternative embodiment of the invention moreparticularly suited for blanket tungsten deposition processes.

FIG. 6C is an enlarged cross-sectional view, similar to FIG. 6, of thesusceptor of an alternative to the embodiment of FIG. 6B.

FIG. 7 is a top view of the susceptor of FIG. 6B, but with the waferremoved.

FIG. 8 is a top view of the susceptor of FIG. 6C, but with the waferremoved.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a CVD module 10 for a wafer processing cluster toolin which are embodied features of the present invention. The module 10includes a frame 11 on a wheeled base 12, which has depending therefroma set of adjustable feet 13 for leveling the module 10 and anchoring themodule 10 to a floor. The module 10 includes a cabinet 14 fixed to theframe 11 that contains flow controllers with connections for inlet linesfor supplying reactant gases to a chemical vapor deposition (CVD)reactor 15, also fixed to the frame 11. The cabinet 14 has associatedwith it other parts of the reactor support system that are not shown,including fluid conduits, valves, pumps, controls, and associatedhardware for the operation of the reactor 15 including the supplies andconnections to supplies of the various reactant gases, inert gases,purging and cleaning gases, and cooling fluids for the reactor 15.

The reactant gases for the main CVD process to be performed with thereactor 15, in the preferred embodiment of the invention, are gases usedfor a blanket tungsten deposition process onto silicon semiconductorwafers and are supplied through lines 16, shown as four in number,connected between the cabinet 14 and the reactor 15. These gasesinclude, for example, tungsten hexafluoride (WF₆), hydrogen (H₂), andsilane (SiH₄). The reactor is, however, also useful for titanium nitridefilms and for many other films that can be applied through a CVDprocess. Also supplied through one of the lines 16 may be inert gas suchas argon. In addition, reactant gas for the plasma cleaning of thechamber 15, such as nitrogen trifluoride (NF₃) gas, is supplied througha gas inlet line 17 connected between the cabinet 14 and the reactor 15.The module 10 also includes one or more vacuum pumps 18, and usually onehigh volume low vacuum pump and one low volume high vacuum pump, forevacuating the reactor 15, for maintaining a vacuum within the reactor15 at the required operating pressure levels, and for exhausting unusedreactant gas, reaction byproducts, cleaning gases and inert gasesflowing through the reactor. A residual gas analyzer port 19 is providedfor monitoring the constituents of the gas.

The reactor 15 includes a susceptor rotating and wafer elevatingmechanism 20 depending from the bottom of the reactor 15. The mainevacuation of the reactor 15 is accomplished through a vacuum outletline 21 connected between the reactor 15 and the vacuum pump or pumpassembly 18 while one or more auxiliary vacuum outlet lines 22 areprovided, connected between the mechanism 20 and the pump assembly 18. Acombined upper electrode electrical terminal and cooling fluid manifoldconnector 23 and a combined lower electrode electrical terminal andcleaning gas connector 24 are also connected between the reactor 15 andthe support systems in the vicinity of cabinet 14.

Referring to FIG. 2, the CVD reactor 15 has sealed within it a reactionchamber 25 enclosed in a housing 26 by which the reactor 15 is mountedthrough rubber vibration absorbing pads 29 to the frame 11 and from thebottom of which the mechanism 20 is supported. The housing 26 ispreferably made of aluminum with a highly polished interior, and isprovided with independent temperature control, both for heating andcooling of the reactor wall, to produce what is sometimes genericallyreferred to as a cold wall reactor, as distinguished from an oven typereactor in which the susceptor is heated by radiant heat from a heatedreactor wall. The housing 26 is preferably fluid cooled, by a suitablefluid such as ethylene glycol or water. In addition, resistance heatingelements (not shown) are also provided in the housing 26 so that thehousing may be heated, or, alternatively or in addition, rod typeheating elements may be provided in the chamber at various locations.One or more of the heating or cooling features may be employed in thesame structure, depending on its intended applications. The heating andcooling of the reactor wall may be zone controlled, and may have boththe heating and cooling active simultaneously for more responsivetemperature regulation and uniformity.

The housing 26 has, at the top thereof, a chamber cover 27, preferablyalso of aluminum, encloses the reaction chamber 25 within. The cover 27is pneumatically sealed against the top of the housing 26, or spacers199 if employed, and may be pneumatically held thereto or may bemechanically secured thereto by screws 28 or clamps. In FIG. 2, thescrews 28 are shown securing spacers 199 to the top of the housing 26.The cover 27 has a reactant gas mixing chamber 30 surrounded by anannular mixing chamber wall which may be formed integrally of thealuminum chamber cover 27 or of a separate material such as a machinableceramic or separate aluminum or other metal piece and secured to theunderside of the chamber cover 27. The mixing chamber wall 31 is capableof being actively cooled, where the process, for example a tungstendeposition process, so requires, by cooling fluid supplied to flowthrough an annular passage 32 formed in the wall 31 to maintain it at atemperature lower than the reaction temperature that is independent ofthat of the housing 26 and that of the chamber cover 27. Like thehousing 26, the mixing chamber wall 31 is also provided with resistanceheating elements (not shown) to heat the wall and the mixing chamber 30where the process so requires, such as for titanium nitride deposition.This annular wall 31 may be made of a thermally nonconductive materialor of a conductive material thermally insulated from the aluminummaterial of the cover 27 to provide greater flexibility in the controlof its temperature. The upper portion of the mixing chamber 30 is closedby a removable cover or top plate 33, preferably of stainless steel,which is sealably connected to the chamber cover 27 by bolts 34 (FIG.4). The chamber housing 26, chamber cover 27 and top plate 33 form asealed vessel enclosing an internal volume that is maintained at avacuum pressure level during operation of the module 10.

The bottom of the gas mixing chamber 30 is closed by a circularshowerhead 35 connected to the bottom of the mixing chamber wall 31. Theshowerhead 35 may be made of aluminum or of a machinable ceramicmaterial and has a highly polished lower surface to retard theabsorption of radiant heat from the higher reaction temperature from thearea of a wafer being processed within the chamber 25. The showerhead 35has, in one acceptable embodiment, a uniform pattern of holes 36therethrough (FIG. 4), preferably arranged in a matrix or an array inplural concentric circles about the center thereof, which lies on avertical axis 37 through the reactor 15. Alternatively, the showerhead15 may be formed of a porous metal or ceramic plate.

A plurality of gas inlet ports 38 (FIG. 4) are provided in the top plate33 to which the gas lines 16 are connected. A rotary wafer supportingsusceptor 40 is provided within the chamber 25. The susceptor 40 lies onthe axis 37 directly beneath the showerhead 35 and is in axial alignmenttherewith. A cleaning gas entry port 41 is mounted to the chamber cover27 and is connected to the cleaning gas input line 17. The RF upperelectrode terminal and cooling water connector 23 is also mounted to thechamber cover 27. The lower electrode RF terminal and cleaning gasconnector 24 are mounted to the side wall of the housing 26. A singlevacuum outlet port 42 is provided in the bottom of the chamber housing26 to which the vacuum outlet line 21 is connected to the pump 18, whichoperates at a pumping rate of from 400-500 liters per second to achievethe wafer processing pressures at between 1 and 100 Torr, reactorcleaning pressures of from 0.1 to 100 mTorr, and wafer transferpressures of 10⁻⁴ Torr within the chamber 25. A gate port 43 is providedin the forward wall of the housing 26 for connection to a transportmodule or wafer handling module of a cluster tool, to and from whichwafers are loaded and unloaded of chambers 25 for processing. The gate43 is approximately in horizontal alignment with an upwardly facingwafer supporting top surface 44 of the susceptor 40 whereupon a wafer issupported for processing with its upwardly facing side disposedhorizontally parallel to and in vertical alignment with the showerhead35. A plurality of ports 45 are provided in horizontal alignment withthe wafer support surface 44 or the housing 26 on opposite sides of thereaction chamber 25 for inserting diagnostic or other instrumentation.

Fixed to the bottom of the housing 26 and aligned with the reactor axis37 is a susceptor drive support frame 47. Rotatably mounted within thedrive support frame 47 is a hollow susceptor drive shaft 50. The driveshaft 50 is mounted to rotate on its axis, which is on the reactor axis37, extends through a hole 51 in the bottom of the reactor housing 26,and is rigidly connected to the bottom of the susceptor 40. At the hole51, the shaft 50 is rotatably supported on a main bearing 52 having itsinner race surrounding the shaft 50 in tight contact therewith and itsouter race fixed to the frame 47 at the bottom of the housing 26. Asecondary bearing 53, connected to the lower end of the frame 47,tightly surrounds and supports the lower end of the drive shaft 50.Secured to the support frame 47 immediately below the bearing 52 andtightly surrounding the shaft 50 is a ferrofluidic seal 54. Theferrofluidic seal 54 has fluid circulated through it at a temperature ofless than 70° C. to prevent the ferrofluid within it from decomposingand losing its magnetic properties due to heat from the shaft 50. Abovethe secondary bearing 53 within the frame 47 and also surrounding theshaft 50 is an electrical slip ring connector 55. The slip ring 55provides electrical connection with the rotating shaft 50 to supplyelectrical energy to the rotating susceptor and receives sensedtemperature signals therefrom. Fixed to the shaft 50 between the seal 54and the slip ring 55 is a drive pulley 56 which is drivably connectedthrough a drive belt 57 with the output of a susceptor rotation drivemotor 58.

At the lower end of the rotating and elevating mechanism 20, fixed tothe bottom of the frame 47, is a wafer lift mechanism 60, illustrated inmore detail in FIG. 3. The lift mechanism 60 includes an outerfluid-tight shell 61 with a hollow interior enclosing the lower end of ahollow and vertical lift tube 62. The tube 62 extends vertically fromthe lift mechanism 60 upwardly through the frame 47 and through thehollow interior of the drive shaft 50, along the axis 37 of the reactor,and into the chamber 25, terminating in the interior of the susceptor40. The tube 62 rotates with the drive shaft 50 and slides axiallytherein a distance of approximately nine millimeters to raise and lowera wafer on the wafer support surface 44 of the susceptor 40 in thereaction chamber 25. The lower end of the tube 62 is fixed to a hubpiece 63 and rotatably supported in a ferrofluidic seal 64, the outersurface of which is fixed in a sleeve 65 which is vertically slidable inthe shell 61. The lower end of the sleeve 65 is linked to a verticalactuator 66 extending through a hole 67 in the bottom of the shell 61 ofa linear action pneumatic lift 66a. Another ferrofluidic seal 68 isprovided near the upper portion of the interior of the shell 61surrounding the tube 62 on the axis 37 adjacent the bottom of the frame47 of the rotating and elevating mechanism 20. As with the ferrofluidicseal 54, the seals 64 and 68 are supplied with fluid that is maintainedat a temperature of 70° C. or less.

A source of helium gas (not shown) is connected to a helium gas inletport 70 at the bottom of the shell 61 of the lift mechanism 60. Theinlet port 70 communicates with a helium inlet channel 71 at the base ofthe hub piece 63, which communicates through the hollow interior thereofwith an axial bore 72 of the tube 62, extending the length thereof, tocommunicate with the channel 176.

A vacuum outlet port 74 is provided in the shell 61 and connects with anelongated hollow tube 73 to apply vacuum in a hollow space 75 within thedrive shaft 50 at the upper end thereof surrounding the tube 62, asillustrated in FIG. 3A. The hollow space 75 extends the length of thedrive shaft 50 and also communicates with the interior of the susceptor40 within the reaction chamber 25. In one embodiment, described in moredetail in connection with FIG. 6B below, the vacuum pressure at the port74 is maintained at a pressure sufficiently lower than that of thechamber 25 to develop a vacuum in the susceptor 40 to operate as avacuum chuck to hold a wafer against the susceptor surface 44 duringprocessing. This vacuum clamping pressure is communicated between thevacuum port 74 and the space 75 at the top of the drive shaft 50 throughan annular column 79 that surrounds the tube 62 and lies within the tube73. In other embodiments that do not use vacuum clamping, the vacuum atport 74 is maintained at a pressure that will develop a vacuum in thesusceptor 40 that is equal to or slightly greater than the pressure inthe chamber 25. In this way, the entry of reactant gases into thesusceptor prevented, as with the embodiment of FIG. 6, described in moredetail below.

The details of the reaction chamber portion of the CVD reactor areillustrated in FIG. 4. The gas mixing chamber 30 is provided with fourconcentric hollow tubular rings 77, one connected to each of the inletports 38, as illustrated in FIGS. 4 and 4A. Each of the rings 77 has aplurality of holes 76 spaced along them and around the axis 37 to allowa uniformly distributed flow of each of the gases from the inlet ports38 and into the gas mixing chamber 30, where they are uniformly mixed,usually at sub-reaction temperature. From the gas mixing chamber 30, theuniformly mixed gas from the various inlet ports 38 flows downwardlythrough the plurality of holes 36 in the showerhead plate 35 parallel tothe axis 37 and perpendicular to the wafer support surface 44 of thesusceptor 40 as represented by arrows 78.

Surrounding the showerhead 35 is an annular plasma cleaning electrode 80mounted on a insulator 81, of teflon or other suitable insulatingmaterial, secured to the aluminum chamber cover 27. This electrode isenergized to generate a plasma for plasma cleaning of the chamber. Theelectrode 80 has an inner surface 82, which is frustoconical in shapeand angled to present a smooth transition from the diameter of theshowerhead 35 to the larger diameter of the chamber housing 26 toprevent turbulence in the downwardly flowing reactant gases. A pluralityof gas inlet orifices 83 are provided around the surface 82 andcommunicate with a cleaning gas passage 84, which is annular in shape,within the electrode 80. The passage 84 communicates with a supply tube85, which connects to the gas inlet 41 to which the cleaning gas inletline 17 is connected.

An annular cooling passage 87 communicates through a tube 88 with acooling liquid and upper electrode connector 23 (which contains bothfeed and return lines, which are not shown, for the cooling liquid).Radio frequency energy (RF) is fed to the electrode 80 through the tube88 from the connector 23. Cooling liquid such as ethylene glycol orwater is separately provided through cooling liquid inlet and returnports 89 to the cooling passage 32 in the mixing chamber wall 31.

A lower plasma cleaning electrode 90 is provided at the base of thechamber 25 mounted to the chamber housing 26 on an electrical insulator91, also of a suitable insulating material. The electrode 90 is in theshape of an annular ring which serves as a gas flow baffle between theprocessing portion of the chamber 25 and a vacuum outlet port 42, asillustrated in FIGS. 4, 4B and 4C. The electrode 90 defines an annulargas flow opening 92 between the electrode 90 and a sleeve 93 fixed tothe base of the housing 26 and surrounding the susceptor drive shaft 50through which the cleaning gas and cleaning by-products pass downwardlyas they are exhausted from the chamber 25. Openings 94, provided aroundthe top of the electrode 90, communicate with an annular passage 95 inthe electrode 90, which in turn communicates with another cleaning gassupply tube 96, which in turn communicates with a cleaning gas inletport 97 in the lower electrode terminal and cleaning gas connector 24.The electrode 90 is electrically connected to a power supply (not shown)that supplies RF energy through the tube 96 to the lower electrode 90from the lower electrode terminal and connector 24. Cleaning gas forplasma cleaning of the interior of the internal components of thechamber 25, such as NF₃ gas, enters through the openings 83 and 94 inthe respective electrodes 80 and 90 and exhausts through the port 42.

Two additional aluminum baffles 101 and 102 are provided between theelectrode 90 and the base of the housing 26. The baffles 101 and 102 arestacked vertically on spacers 104 at the base of the housing 26 andsecured thereto by a plurality of bolts 105. The upper one of thesebaffles 101 is disc shaped and extends from the sleeve 93 outwardly.defining a space 106 annularly around the side wall of the housing 26.The lower one of these baffles 102 is also disc shaped and extends fromthe side wall of the housing 26 inwardly to define a space 107 annularlyaround the sleeve 93.

The susceptor 40 has an exterior side surface 110 which is smoothlycontoured to minimize turbulence in the flow of reacting gases withinthe chamber 25. The surface 110, at its widest part, defines a space oropening between the susceptor 40 and the side wall of the chamberhousing 26. The horizontal cross-sectional area of the opening 111 isgreater than that of the opening or space 92 defined by the electrode90, which is in turn greater than the horizontal area of the space 106defined by the baffle 101, which is in turn greater than the horizontalcross-sectional area of the opening 107 defined by the baffle 102. Theratio of these areas provides a pressure gradient, when the reacting gasis flowing through the chamber 25, which minimizes turbulence andprovides for the uniformity around the susceptor 40 of the gas flowthrough the chamber 25 to the single vacuum outlet port 42. This flow isillustrated by the arrows 112, 113, 114 and 115.

In certain situations, it may be desirable to incorporate an alternativestructure to enhance the reliability and extend the life of the mainbearing 52, as for example, where the life of the seal shortens timebetween scheduled reactor maintenance. Such an alternative to thestructure at the point where the shaft 50 passes through the base of thehousing 26 is illustrated in FIG. 5. In this alternative, the base ofthe housing 26 is provided a nitrogen gas inlet port 117 and nitrogengas vacuum outlet port 118 (broken line) through which nitrogen gas iscaused to flow in a space 120 between the susceptor drive shaft 50 andthe sleeve 93 as illustrated in FIG. 5. Nitrogen gas flowing in throughthe port 117 is injected into an annular passage 121 surrounding theshaft 50 at the base of the sleeve 93, flows in the direction of thearrow 122 into an annular passage 123 above the passage 121 in thesleeve 93 and then out the port 118. Similarly, nitrogen gas flows intothe third annular space 124, above the passage 123, from the port 117.Part of the gas from the passage 124 flows in the direction of the arrow125 to the passage 123 and out the port 118 while a portion of the gasfrom the passage 124 flows in the direction of the arrow 126 into thespace 120 and then in the direction of the arrow 127 into the reactionchamber approximately in the vicinity of the space or opening 92 aroundthe outer edge of the lower cleaning electrode 90. This outward flow ofnitrogen gas in the vicinity of the arrow 127 prevents the entry of thereactant gases into the space 120 during the reaction process. It alsoprevents particles and other contamination from entering the space 127.

The susceptor 40 is illustrated in two embodiments in FIGS. 6 and 6B.Each of these embodiments includes some of the alternative features thatmay be desirable depending on the application. FIG. 6A is across-sectional view that shows the placement of features that appear inone or both of the embodiments of FIGS. 6 or 6B. The susceptor 40 of theembodiment of FIG. 6 utilizes electrostatic wafer clamping, anelectrically insulating wafer supporting surface, an insulating ringaround the outer edge of the wafer support surface, RTDs for temperaturesensing in the wafer support, an inert purge gas around the rim of thewafer, and an inert gas within the susceptor at a pressure at orslightly greater than the pressure within the chamber 25. Many of thefeatures of the embodiment of FIG. 6 are more suitable for selectivedeposition of certain materials such as tungsten, as explained below.

The susceptor 40 of the embodiment of FIG. 6B utilizes vacuum waferclamping, a metallic wafer support and a metallic scavenger ring aroundthe outside of the wafer support, thermocouple temperature sensing inthe wafer support, and an inert gas within the susceptor at a pressureless than that of the chamber 25. Many of the features of the embodimentof FIG. 6B are suitable for blanket deposition of materials such astitanium nitride and tungsten, also as explained below.

The susceptor 40 of FIGS. 6, 6A and 6B is provided with a thin outermetal wall 130 which is mounted by bolts 131 to the top of the driveshaft 50, as is better illustrated in FIG. 6, which illustrates thesusceptor structure of one embodiment. The wall 130 is of a high heatconductive material, for example a metal such as aluminum, and has athin cross-section to minimize the flow of heat from the upper portionof the susceptor to the shaft 50. The wall 130, which has as its outersurface the surface 110 of the susceptor 40, has a highly polishedreflective inner surface 132 to reflect, and thus minimize theabsorption of, heat from the downward facing surface 129 of the heatedupper portion of the susceptor 40 and through the hollow inner space 135of the susceptor 40. The outer surface 110 is provided with a dullfinish to maximize heat radiation from the wall 130.

At the base of the susceptor wall 130, formed integrally therewith, is adownwardly extending collar 136 which surrounds the shaft 50 and isspaced therefrom to leave a small cylindrical gap 137 between the collar136 and the shaft 50 to reduce direct heat conduction from the wall 130to the shaft 50. Projecting inwardly from the collar 136 and formedintegrally with the wall 130 is a susceptor mounting flange 138. At theupper end of the shaft 50 is an annular upwardly projecting shoulder 140on which the flange 138, and thus the wall 130, is supported to therebysupport the susceptor 140 for rotation with the shaft 50. The shoulder140 presents a small contact area with the flange 138 to minimizethermal contact therebetween and minimize heat transfer from thesusceptor wall 130 to the shaft 50. The upwardly extending shoulder 140defines small gap 141 between the top of the shaft 50 and the flange 138to further reduce direct heat conduction between the inner portion ofthe flange 138 and the top of the shaft 50.

Through the disc 142 extend bolts 131, which are threaded into the topof the shaft 50. In the embodiment of FIG. 6, the flange 138 has anupwardly extending shoulders 143 formed thereon to space the disc 142from the flange 138, to present minimal contact area therebetween toreduce thermal conduction, and to define a further gap 144 between theflange 138 and wall 130 and the disc 142. These shoulders 143 areomitted from the alternative embodiment of FIG. 6B. Additional thermalinsulation between the susceptor 40 and the shaft 50 may be achieved, ifnecessary, by providing a layer of insulating material, such asinsulating washers or spacers, between the flange 138 and the shaft 50.A seal 145 (shown as an O-ring in FIG. 6 and as a soft metal seal inFIG. 6A) is provided in an annular space 146 formed around the outsideof the upper end of the shaft 50 between the shaft 50 and the collar 136and flange 138 of the susceptor wall 130. A plurality of holes 147 isprovided through the disc 142 to communicate between hollow space 75within the shaft 50 and the space 135 within the susceptor 40 to providefor the maintenance of a vacuum within the space 135 at approximately 10Torr.

Projecting upwardly from the top of the disc 142 on the axis 27 is avertical hub portion 149 which has an interior hole through which thetop end of the hollow lift rod 62 extends.

The upper portion of the susceptor 40 includes a wafer support structure150 formed of a pair of discs including an upper disc 151 and a lowerdisc 152.

In this embodiment of FIG. 6, the lower disc 152 is supported at itsouter edge on an inwardly extending support flange 153 formed integrallyof the susceptor wall 130 and having an annular channel 154 on the uppersurface thereof which contains a seal 155 to isolate an annular space156, formed between the outer rim of the disc 152 and the wall 130, andthe space 135. In this embodiment, the space 156 is a heliumdistribution channel which communicates through a circumferentiallyspaced set of ducts 157 with helium supply tubes 158 which extendradially from and mechanically connect to the top end of the tube 62above the top surface of the hub 149. With this arrangement, helium gasis caused to flow upwardly through the tube 62 and outwardly through thetubes 158 and up through the ducts 157 and into the channel 156. Thetubes 158 have flexible mid sections 159 to permit vertical movement ofthe lift rod 62 with respect to the wall 130 while the outer ends of thetubes 158 are stationary with respect to the flange 153. This helium gasis maintained at a separately regulated pressure to produce a pressurethat is equal to or very slightly greater, at the gap 166, than thepressure of the reactant gases flowing immediately above the gap 166.

Also in the embodiment of FIG. 6, the upper disc 151 is supported on thetop of the lower disc 152 and has an upper wafer supporting surface 160thereon, which forms part of the upper susceptor surface 44, theremainder of which is formed by an upper surface 161 of an annularsusceptor lip piece 162. As illustrated in FIGS. 4B and 6, the lip piece162 is bolted to the top of the susceptor wall 130 by bolts 163. The lip162 is shaped such that its upper surface 161 lies in the samehorizontal plane as the upper surface 164 of a wafer 165 when the wafer165 is supported on the surface 160. A small annular gap 166 around theperimeter of the wafer 165 provides sufficient clearance between the lip162 and the wafer 165 to allow for thermal expansion of the wafer andfor wafer diameter tolerances. The relationship of the lip 162 to thewafer 165 thus avoids turbulence in the flow of gas across the surfaceof the wafer 165 and the upper surface 44 of the susceptor 40.

In the embodiment of FIG. 6, the disc 151 rests on a seal 171 in achannel 172 in the top of the lower disc 152, and a further seal 173 isprovided in a channel 174 at the top end of the susceptor wall 130between the lip 162 and the susceptor wall 130.

In the susceptor of the embodiment of FIG. 6, through the upper portionof the susceptor wall 130 and the lip 162 is a circular array of ducts175 which communicate between the helium channel 156 and a peripheralchannel 176 surrounding the upper disc 151 beneath the lip 162. Thisprovides a path for helium gas to flow from the annular space or channel156, ducts 175, channel 176 and outwardly through the gap 166 around theperimeter of the wafer 165 to prevent the flow of reactant gasesdownwardly into the gap 166 around the perimeter of the wafer 165 and toprovide for smooth flow of the reactant gas outwardly across the surface164 of the wafer 165 and the surface 161 of the lip 162. The outer edge178 of the lip 162 is rounded to further avoid turbulence as the gasflows around the edge of the susceptor 40.

For uses such as blanket deposition of tungsten, the support structure150 and the lip 162 are preferably made of Monel, which resistssputtering during plasma cleaning with NF₃ better than do some othermetals. In such processes, the lip 162 serves as a scavenger for unusedreactant gases. For selective deposition processes, the disc 151 and thelip 162 are made of a material on which the tungsten to be depositedwill not nucleate, as nucleation of the material on the susceptorsurfaces adjacent the wafer causes film to deposit on the wafer inblanket fashion near such surfaces.

The susceptor of the embodiment of FIG. 6 includes features suitable forthe selective tungsten deposition process. The discs 151 and 152 of thisembodiment are made of an insulating material such as graphite, to bothprevent nucleation onto the support and to support a charge forelectrostatic clamping of the wafer to the susceptor. For selectivedeposition, because it is frequently desirable to operate the process atpressures of 1 Torr or less, vacuum clamping will not be effective. Theabsence of vacuum clamping also renders the edge purging featureprovided by injecting helium into the cavity 176 more effective, as thisfeature, without elaborate sealing techniques, may cause helium gas toflow below the wafer and destroy the pressure differential needed forvacuum clamping of the wafer, or may have the counterproductive effectof facilitating the flow of reactant gas beneath the wafer and into thespace 135 within the susceptor.

In the embodiment of FIG. 6, a plurality of preferably three lift pins184 are provided, each slidable in holes 181 through the disks 151 and152 of the susceptor 40. The holes 181 are no larger than necessary toallow the pins 184 to slide, and to otherwise minimize flow of gasthrough the holes 181 between the back of the wafer and the space 135within the susceptor.

The features of the susceptor of the embodiment of FIG. 6B are moresuitable for the deposition of blanket tungsten, and accordingly, theremainder of the structure of the susceptor 40 will be described inconnection with the embodiment of this figure.

Referring to FIG. 6B, the discs 151 and 152 may be made of a materialsuch as Monel. Within the disc 151 may alternatively be provided ducts180 (shown by phantom lines), if desired, to distribute helium gasacross the back surface of the wafer 165 for thermal gas conductionbetween the disc 151 and the wafer 165. These ducts 180 are remote fromthe edge space 166 around the rim of the wafer 165 so that the flow ofreactant gas into the space behind the wafer is not enhanced thereby.

The ducts 180 are in the form of grooves in the upper surface of thedisk 150 as illustrated in FIG. 7. They include three concentriccircular grooves 180a, 180b and 180c interconnected by three sets ofradial grooves spaced 120° apart, including grooves 181a, whichinterconnect on the axis of the susceptor 37 and extend to the innermostcircular groove 180a, radial grooves 181b which interconnect circulargroove 180a, the intermediate one of the circular grooves 180b and arespective one of the holes 182, and radial grooves 181c, whichinterconnect circular groove 180b with the outermost circular groove180c.

Gas at the backside of the wafer 165 is maintained at a pressure lowerthan in the reaction chamber 25 by way of oversize vertical holes 182that, unlike the embodiment of FIG. 6, fit loosely around the lift pins184 and thus communicate between the back of the wafer 165 and the space135 within the susceptor 40 to provide for vacuum clamping of the wafer165 to the surface 160. The helium gas that fills the space 135 ishelium that is maintained at a separately regulated pressure through theopenings 147 in the disc 142 that communicate between the space 135 andthe space 75 at the top of the drive shaft 50. Helium is supplied intothe space 135, in the embodiment of FIG. 6B, through the short tube158a. The vacuum clamping pressure may be maintained in the space 135 atapproximately 10 Torr where reaction pressure in the reaction space ofthe chamber 25 above the wafer 165, for blanket tungsten CVD processes,is at approximately 50 to 60 Torr.

With processes such as selective tungsten CVD, which may be performed at0.1 to 5.0 Torr, rather than vacuum clamping of the wafer, otherclamping means such as electrostatic clamping, as illustrated in theFIG. 6 embodiment, may be preferable, but some helium should still beprovided behind the wafer, at or very slightly above the reactionchamber pressure to enhance heat transfer between the wafer and the disk151.

In the embodiment of FIG. 6B, the upper disk 151 extends over the top ofthe susceptor wall 130 and is bolted directly thereto by recessed bolts168, compressing a flat soft metalic seal 169 between the disk 151 andthe susceptor wall 130. An alternate form 170 of the lip 162 is fastenedby countersunk screws to the top of disk 151, covering the screws 168and mounted flush so as to form a continuous surface with the topsurface 164 of the wafer 44 and the outer surface 110 of the susceptorwall 130. This form of lip 170 is most suitable when it is made of ametalic material such as Monel.

In the susceptor of both the embodiments of FIGS. 6 and 68, mounted tothe top of the lift rod 62 above the tubes 158 and directly above thehub or bushing 149 of the disc 142 is a horizontal table 183 which movesupward and downward with the lift rod 62. Extending upwardly from theperiphery of the table 183 through the holes 181 (FIG. 6) or 182 (FIG.68) is a plurality of preferably three lift pins 184 which, whenelevated, will contact the underside of the wafer 165 to lift it fromthe surface 160 or lower into the holes 181 or 182 (the positionillustrated in FIGS. 6 and 68) to lower the wafer 165 onto the surface160. At the upward position of the table 183, the wafer 165 will be inposition for transfer into and out of the chamber through the gate port43 (FIG. 4), and at the downward position of the table 183 at which thewafer 165 is lowered onto the surface 160, the wafer is in position forprocessing.

Also in the susceptor of both the embodiments of FIGS. 6 and 6B, thereis mounted between the discs 151 and 152 a resistance heater 185 whichincludes a central circular element 186, an intermediate annular element187 and an outer annular element 188, each providing a plurality ofseparately controllable heater zones at various radii on the wafersupport 150. In the embodiment of FIG. 6, each of the zones is providedwith a temperature sensing element 191, 192 and 193 of the RTD orthermocouple type respectively corresponding to the areas heated by theelements 186, 187 and 188. Each of the elements is provided with aspring loaded electrical contact assembly 195 (only one of which isshown in FIG. 6 with respect the intermediate element 187). Two contactsare provided for each of the heating elements 186, 187 and 188.Electrical conductors 198 for these elements and for the sensors 191,192 and 193 extend downwardly through the shaft 50 to make electricalconnection through the slip ring 55 (FIG. 2) with power supplies andcontrol circuits.

In the embodiment of FIG. 6B, three temperature sensing elements 189 ofthe thermocouple type (only one of which is shown in FIG. 6B, extendthrough holes in the heaters 186, 187 and 188 to recesses in thebackside of disc 151, one directly overlying each of the heater zones.Readings from these thermocouples are fed back to a heater controller(not shown) to maintain uniform temperature on the disk 151. Each of thethermocouples 189 connects to the controller through a wire in the shaft50 at a bracket mounted connector 190 on the wall 130 and in the space135. The electric connectors to the heater terminals are illustrated inthe FIG. 6B embodiment in their preferred form 196, recessed into thelower surface 129 of the lower plate 152 and connected to the leads 198with ceramic screws 194.

The entire assembly as shown in FIGS. 6 and 6B, with the exception ofthe sleeve 93 rotates at from 500 to 1500 rpm to minimize the thicknessof the boundary layer above the surface 164, enabling the process gas toreach the wafer faster and the byproducts from the CVD process to escapemore readily from the wafer surface 164. The flow is illustrated by thearrows 78 and 197 in FIG. 4. Such flow forms a stagnation point at thecenter 198 on the axis 37 as it intersects the surface 164 of the wafer165. The lip 162 is provided with the surface 161 of a substantialupwardly facing area to serve as a scavenger, when made of a material onwhich tungsten nucleates and used in a blanket deposition process, forunused reactant gases such as tungsten hexafluoride, thereby minimizingthe amount of tungsten hexafluoride being exhausted from the chamber 25.This lip 162 is removable and replaceable with a lip having an inwardlyextending portion 167 having a different inside diameter to therebyaccommodate wafers 165 of differing sizes.

FIG. 6C illustrates a further alternative embodiment to the susceptoralternatives in FIGS. 6, 6B and 6C. The embodiment of FIG. 6C is similarin most respects to that of FIG. 6B, with some modifications, and alsoincludes a modified form of the wafer edge purge feature of theembodiment of FIG. 6. As shown in FIG. 8, the embodiment of FIG. 6Cincludes the grooves 180a, 180b, 181a and 181b of FIG. 6B. However, thegrooves 181c are eliminated, and the groove 180c functionally replacesthe annular groove or channel 176 of FIG. 6, but at a position inboardof the edge or rim of the wafer 165. This groove 181c, in the embodimentof FIG. 6C, is connected to a separately regulated source of gas fromthe bore 72 of the tube 62 by the tubes 158, which respectivelycommunicate through a pair of rigid tubes 158a with a respective pair ofports 158b, carried by bored plugs, with a respective pair of radiallyoriented ports 158c. This gas is supplied at a pressure of slightlygreater than the pressure in the chamber 25, for example fromapproximately 0.5 to 1.0 Torr greater, which is lower than the pressurein the grooves 180a, 180b, 181a and 181b of typically 10 Torr. The gasmay be an inert gas such as helium, or a reactive gas that will cleandeposits from the CVD process that might form in the space 166 betweenthe wafer 165 and the lip 162 or 170, such as NF₃ in the case wheretungsten is being applied.

Additionally, optimal reactant gas flow on the surface of the wafer isachieved by varying the spacing between the gas showerhead 35 and thesusceptor 40. Provision for this is made by addition of one or morespacer rings, such as spacer ring 199, between the top edge of thereactor housing 26 and the chamber cover 27 (FIG. 2).

The operation of the module 10 described above for the blanket andselective deposition of tungsten onto semiconductor silicon wafers isdescribed in detail in the copending and commonly assigned patentapplication Robert F. Foster and Helen E. Rebenne entitled METHODS OFCHEMICAL VAPOR DEPOSITION (CVD) OF FILMS ON PATTERNED WAFER SUBSTRATESfiled on even date herewith, hereby expressly incorporated herein byreference.

While the above described embodiments of the invention relate toprocessors of the CVD type, the rotating disk susceptor, gas flow,temperature maintenance and other features of the invention are usefulin connection with other types of processes, especially where rapid anduniform transporting of vapor substances to and from the wafer surfaceis desired. For example, in connection with the deposition of titaniumnitride films, a degassing process is preferably performed in a separatemodule prior to the TiN deposition. In such a process, water that hasbeen absorbed into the wafer, as, for example, might have been absorbedinto a phosphosilicate glass (PSG) or borophosphosilicate glass (BPSG)film deposited onto the wafer prior to the TiN process, is removed byheating the wafer. Also, subsequent to a TiN film deposition, chlorinemay be removed by an annealing process in a separate module. In suchprocesses, a separate dedicated processing module as described above maybe used with, for example, argon or nitrogen gas in the performance of apreheating or degassing process, while another dedicated similar modulemay be used with, for example, ammonia in the performance of anannealing process. In both applications, such a module will function thesame as the CVD module described above except that, instead of materialbeing added to the substrate as is done in a CVD process, material isremoved from the substrate. The benefits of the rotating disk and otherfeatures of the invention nonetheless apply to such processes. Thesebenefits include a uniform boundary layer that is thinned by rotation ofthe susceptor, which in turn results in a faster water or chlorineremoval rate and a more uniform removal rate across the surface of thewafer. Further, the radially outward flow of gas contributes to theflushing of the water, chlorine or other substance away from the surfaceof the wafer, enhancing the efficiency of the removal. This preventsdesorbed material from being redeposited onto the surface of the wafer.

In applying principles of the invention to degas and anneal modules, notall of the structure desirable for CVD applications in the abovedescribed embodiments is necessary. For example, the RF cleaningelectrodes 80 and 90 may be eliminated, as well as the power connectionsand supplies powering them. Further, only one baffle at the bottom ofthe chamber 25 is usually sufficient. The number of gas supplies andassociated equipment may, of course, be limited to that needed for theapplication. Further, since such processes are basically heat treatingprocesses, the chamber housing 26 is preferably insulated from theoutside.

In order to achieve optimum processing uniformity with the rotatingsusceptor described in embodiments above, the process should be operatedunder conditions dictated by the rotation rate. In CVD applications,this optimization will achieve the highest deposition rate and reactantconversion without sacrificing film uniformity or properties. To producethese conditions, the total mass flow rate of gas flowing radiallyoutward on the susceptor surface is matched by an equal mass flow rateof gas flowing along the axis from the showerhead toward and against thesusceptor surface. The downward flow rate is furnished and controlled bythe rate of injection of the inlet gas. If the inlet gas flow rate istoo small, the susceptor becomes starved for fluid, while if the inletgas flow rate is too high, fluid backs up near the susceptor surface. Ineither case, the velocity profile will not be of the proper shape togive a uniform boundary layer thickness near the susceptor surface andhence the benefits of rotation will not be fully realized. At a giventemperature, pressure, inlet gas composition, and susceptor rotationrate, one inlet gas flow rate or a narrow range of inlet gas flow ratesgives optimum operation. This flow rate is commonly referred to as the"matched flow rate" for the given set of conditions. They may bedetermined theoretically or by experimentation for each process and eachreactor, and preferably, first theoretically and then verified or finetuned experimentally. For blanket and selective tungsten CVD, an inletgas flow rate will fall generally within the range of from 0.5 slpm to5.0 slpm for the temperatures, pressures, gas composition and rotationalspeeds discussed above. For example, for blanket tungsten deposition,0.1 slpm of WF₆ and 2.0 slpm for H₂, for a total flow of 2.1 slpm, hasbeen found preferable for 425° C., 80 Tort and 750 RPM. For selectivetungsten CVD, 0.1 slpm of SiH₄, 0.15 slpm for WF₆ and 2.75 slpm for H₂,for a total flow of 3.0 slpm, has been found preferable for 280° C., 5Torr, and 250 RPM. Generally, flow rate must be increased whentemperature, rotational speed or viscosity are increased, or whenpressure is decreased, when the other parameters are held constant.

While the above detailed description sets forth a preferred embodimentof the invention, it will be apparent to those skilled in the art thatvariations and modifications can be made without departing from theprinciples of the invention. The principles of the present inventioninclude several concepts most useful for CVD, and useful for other waferprocessing applications, particularly those in which material is to betransferred from a gas to a wafer, or from a wafer to a gas. Variousdetails of the reactor of the described embodiments may be modified indesign, and may be combined in the same structure. For example, thelower plasma electrode has been described and combined with structureconstituting a baffle. Similarly, the upper plasma electrode, whileprovided in separate structure in the preferred embodiment, may becombined with or incorporated into the showerhead. Accordingly, thesubject matter of the invention is intended to be limited only by thefollowing claims:

We claim:
 1. An apparatus for processing semiconductor waferscomprising:a sealed vessel enclosing an interior volume, the vesselhaving exhaust means having an exhaust port connected to one end of thevolume for maintaining the volume at a vacuum pressure level; asusceptor supported on an axis in a processing space in the interiorvolume of the vessel, the susceptor having a wafer supporting surfacethereon oriented perpendicular to the axis; gas introduction meansdisposed at one end of the volume opposite the susceptor from theexhaust means for introducing gas supplied thereto into the interiorvolume of the vessel; means for supplying at least one processing gas tothe gas introduction means; the introduction means being disposedparallel to the wafer supporting surface and generally centered on theaxis and spaced from the wafer supporting surface for directing a flowof the processing gas from the introduction means into the processingspace parallel to the axis, toward and perpendicular to the wafersupporting surface of the susceptor; means carried by the susceptor forholding the wafer to the supporting surface for processing with asurface thereof centered thereon and facing the introduction means; andstationary annular opposed gas directing surfaces distributed uniformlyaround the axis for directing the gas in a smooth non-turbulent flowfrom the gas introduction means, across the wafer and past the susceptorto the exhaust means, the gas directing surfaces forming baffle means,surrounding the axis and axially positioned between the wafer supportingsurface of the susceptor and the exhaust means, for facilitating theexhausting of gas through the exhaust port without creating turbulenceinside the processing space.
 2. The apparatus of claim 1 wherein:thebaffle means comprises a plurality of axially spaced baffles eachdefining an annular passage around the axis.
 3. The apparatus of claim 2wherein:the passages have cross-sectional areas that decrease with theirproximity to the exhaust port.
 4. The apparatus of claim 3 wherein:theprocessing space is bounded by a vessel housing and the susceptor has anouter wall spaced from the housing to form a passage therebetween thathas a cross-sectional area greater than those of the passages.
 5. Theapparatus of claim 1 wherein:the apparatus is a CVD reactor and theprocessing gas includes at least one reactant gas; the susceptor has anannular lip surrounding the wafer support surface and having an insideopening dimension so that the lip is close to a circular outer edge of awafer supported on the wafer support surface, the lip having outwardlyfacing surface means positioned flush with the surface of the wafer forreducing turbulence and radial thermal gradients in the wafer near theedge thereof; and the apparatus further comprises means for introducingnon-reactive gas between the lip and the edge of the wafer so as toprevent flow of reactant gas from the wafer between the lip and the edgeof the wafer, to thereby reduce deposition on the edge and bottom marginof the wafer.
 6. The apparatus of claim 5 wherein:the non-reactive gasintroducing means includes a supply of helium gas.
 7. The apparatus ofclaim 1 wherein:the gas introduction means is downwardly facing fordirecting the flow of processing gas downward from the gas introductionmeans into the processing space from above, and the wafer supportingsurface is upwardly facing.
 8. The apparatus of claim 1 wherein:theapparatus is a CVD reactor and the processing gas includes at least onereactant gas; the wafer holding means includes means for providing anon-reacting gas between the wafer and the wafer supporting surface andfor maintaining the non-reacting gas between the wafer and the wafersupporting surface at a vacuum pressure that is below the pressure inthe reaction space, to facilitate the holding of the wafer to thesusceptor; the non-reacting gas between the wafer and the wafersupporting surface being maintained at a pressure sufficient to provideheat transfer by gas conduction between the wafer and the wafer supportsurface; and the non-reacting gas between the wafer and the wafersupporting surface being maintained at a pressure of from approximatelyat least 1 Torr and a higher pressure which is not more than iseffective to conduct heat between the wafer and the supporting surfaceby gas conduction.
 9. The apparatus of claim 1 further comprising:asource of non-reacting gas; the wafer holding means includes means forcausing the non-reacting gas to flow from the source thereof and betweenthe wafer and the wafer supporting surface and for maintaining a vacuumpressure between the wafer and the wafer supporting surface that isbelow the pressure in the reaction space, to facilitate the holding ofthe wafer to the susceptor.
 10. The apparatus of claim 9 wherein:thenon-reacting gas between the wafer and the wafer supporting surface ismaintained at a pressure sufficient to provide heat transfer by gasconduction between the wafer and the wafer support surface.
 11. Theapparatus of claim 1 further comprising:means for providing non-reactinggas between the wafer and the wafer supporting surface at a pressuresufficient to provide heat transfer by gas conduction between the waferand the wafer support surface.
 12. An apparatus for processingsemiconductor wafers comprising:a sealed vessel enclosing an interiorvolume, the vessel having exhaust means having an exhaust port connectedto one end of the volume for maintaining the volume at a vacuum pressurelevel; a susceptor supported on an axis in a processing space in theinterior volume of the vessel, the susceptor having a wafer supportingsurface thereon oriented perpendicular to the axis; gas introductionmeans disposed at one end of the volume opposite the susceptor from theexhaust means for introducing gas supplied thereto into the interiorvolume of the vessel; means for supplying at least one processing gas tothe gas introduction means; the introduction means being disposedparallel to the wafer supporting surface and generally centered on theaxis and spaced from the wafer supporting surface for directing a flowof the processing gas from the introduction means into the processingspace parallel to the axis, toward and perpendicular to the wafersupporting surface of the susceptor; means carried by the susceptor forholding the wafer to the supporting surface for processing with asurface thereof centered thereon and facing the introduction means; andgas directing surfaces distributed uniformly around the axis fordirecting the gas in a smooth non-turbulent flow from the gasintroduction means, across the wafer and past the susceptor to theexhaust means, the gas directing surfaces including a smoothly contouredexternally curved shaped outer wall means on the susceptor forminimizing turbulence in the flow of gas in the processing space.
 13. Anapparatus for processing semiconductor wafers comprising:a sealed vesselenclosing an interior volume, the vessel having exhaust means connectedto one end of the volume for maintaining the volume at a vacuum pressurelevel; a susceptor supported on an axis in a processing space in theinterior volume of the vessel, the susceptor having a wafer supportingsurface thereon oriented perpendicular to the axis; gas introductionmeans disposed at one end of the volume opposite the susceptor from theexhaust means for introducing gas supplied thereto into the interiorvolume of the vessel; means for supplying at least one processing gas tothe gas introduction means; the introduction means being disposedparallel to the wafer supporting surface and generally centered on theaxis and spaced from the wafer supporting surface for directing a flowof the processing gas from the introduction means into the processingspace parallel to the axis, toward and perpendicular to the wafersupporting surface of the susceptor; means carried by the susceptor forholding the wafer to the supporting surface for processing with asurface thereof centered thereon and facing the introduction means; andgas directing surfaces distributed uniformly around the axis fordirecting the gas in a smooth non-turbulent flow from the gasintroduction means, across the wafer and past the susceptor to theexhaust means, the gas directing surfaces including an annular lip onthe susceptor surrounding the wafer supporting surface, the lip havingan inside opening dimension which is close to a circular outer edge of awafer supported on the wafer supporting surface, the lip having outersurface means positioned to be flush with the surface of the wafer forreducing turbulence and thermal gradients in the wafer near the edgethereof in an outward direction perpendicular to the axis.
 14. Theapparatus of claim 13 wherein:the apparatus is a CVD reactor and theprocessing gas includes at least one reactant gas; and the surface meansof the lip has an area sufficiently substantial to serve as a scavengerfor unused reactant gas and is of a material on which the reactant gasnucleates to deposit a coating thereon, thereby reducing the amount ofreactant flowing beyond the lip into the reaction space.
 15. Theapparatus of claim 13 wherein:the apparatus is a CVD reactor and theprocessing gas includes at least one reactant gas; and the lip and thewafer supporting surface are formed of a material on which the reactantgas does not nucleate in an acceptable length of time for deposition ofa coating thereon, to thereby facilitate selective deposition of thecoating on the wafer.
 16. The apparatus of claim 13 wherein:the lip hasa smoothly contoured circular outer rim means for reducing turbulence inthe processing space.
 17. The apparatus of claim 13 wherein the lip isremoveably attached to the susceptor.
 18. The apparatus of claim 13wherein:the annular lip is a first annular lip and the inside openingthereof is dimensioned so that the lip is close to a circular outer edgeof a wafer of a first size supported on the wafer supporting surface,and the outer surface means being flush with the surface of the wafer ofthe first size for reducing turbulence and radial thermal gradients inthe wafer near the edge thereof; the apparatus further comprises asecond lip, annular in shape, having an inside opening dimensioned sothat the lip is close to a circular outer edge of a wafer of a secondsize that differs from the first size so that the susceptor accommodateswafers of different sizes, the second lip having outer surface meansflush with the surface of the wafer of the second size for reducingturbulence and radial thermal gradients in the wafer near the edgethereof; and at least said second lip being removeably and attachable tothe susceptor.
 19. The apparatus of claim 13 wherein:the gasintroduction means is downwardly facing for directing the flow ofprocessing gas downward from the gas introduction means into theprocessing space from above, and the wafer supporting surface isupwardly facing.
 20. An apparatus for processing semiconductor waferscomprising:a sealed vessel having a sealed housing enclosing an interiorvolume, the vessel having exhaust means connected to one end of thevolume for maintaining the volume at a vacuum pressure level, thehousing including a susceptor mount; a susceptor supported on a mount ina processing space in the interior volume of the vessel, the susceptorhaving a wafer supporting surface thereon; means for heating a waferheld on the supporting surface to a processing temperature; and meansfor inhibiting the flow of heat between the susceptor and the housing.21. An apparatus for processing semiconductor wafers comprising:a sealedvessel having a sealed housing enclosing an interior volume, the vesselhaving exhaust means connected to one end of the volume for maintainingthe volume at a vacuum pressure level, the housing including a susceptormount; a susceptor supported on the susceptor amount in a processingspace in the interior volume of the vessel, the susceptor having a wafersupporting surface thereon and a hollow interior bounded by a susceptorwall the inside of which has means including a highly reflective surfacethereon for reducing heat transfer to the wafer supporting surface ofthe susceptor and to the susceptor mount; and means for heating a waferheld on the supporting surface to a processing temperature.
 22. Anapparatus for processing semiconductor wafers comprising:a sealed vesselhaving a sealed housing enclosing an interior volume, the vessel havingexhaust means connected to one end of the volume for maintaining thevolume at a vacuum pressure level, the housing including a susceptormount; a susceptor supported on the susceptor mount in a processingspace in the interior volume of the vessel, the susceptor having a wafersupporting surface thereon and an interior bounded by a susceptor wallthe outside of which has means including a low reflectivity surfacethereon to increase radiation of heat away from the susceptor, therebyreducing heat transfer between the wafer supporting surface and themount; and means for heating a wafer held on the supporting surface to aprocessing temperature.
 23. An apparatus for processing semiconductorwafers comprising:a sealed vessel having a sealed housing enclosing aninterior volume, the vessel having exhaust means connected to one end ofthe volume for maintaining the volume at a vacuum pressure level, thehousing including a susceptor mount; a susceptor supported on thesusceptor mount in a processing space in the interior volume of thevessel, the susceptor having a wafer supporting surface thereon and ahollow interior bounded by means including a thin susceptor wall forreducing heat transfer between the wafer supporting surface and themount; and means for heating a wafer held on the supporting surface to aprocessing temperature.
 24. An apparatus for processing semiconductorwafers comprising:a sealed vessel having a sealed housing enclosing aninterior volume, the vessel having exhaust means connected to one end ofthe volume for maintaining the volume at a vacuum pressure level, thehousing including a susceptor mount; a susceptor supported on a mount ina processing space in the interior volume of the vessel, the susceptorhaving a wafer supporting surface thereon and means including lowthermal conductivity mounting material between the susceptor and themount for securing the susceptor to the mount and for providing athermal block to reduce heat transfer between the wafer supportingsurface and the mount; and means for heating a wafer held on thesupporting surface to a processing temperature.
 25. An apparatus forprocessing semiconductor wafers comprising:a sealed vessel having asealed housing enclosing an interior volume, the vessel having exhaustmeans connected to one end of the volume for maintaining the volume at avacuum pressure level, the housing including a susceptor mount; asusceptor supported on the susceptor mount in a processing space in theinterior volume of the vessel, the susceptor having a wafer supportingsurface thereon and means including a hollow interior bounded by asusceptor wall; the mount including first mounting structure on thesusceptor wall and second mounting structure secured to the mount, thefirst and second mounting structure being in contact with each other,and the second mounting structure having a reduced cross-sectional areapresenting a thermal contact surface having an area sufficiently smallto minimize heat transfer at the interface of the first and secondmounting structure; and means for heating a wafer held on the supportingsurface to a processing temperature.
 26. An apparatus for processingsemiconductor wafers comprising:a sealed vessel enclosing an interiorvolume, the vessel having an exhaust port connectable to a pumpeffective to maintain the volume at a vacuum pressure level; a susceptorsupported on an axis in a processing space in the interior volume of thevessel, the susceptor having a wafer supporting surface thereon orientedperpendicular to the axis; a gas introduction inlet disposed at one endof the volume opposite the susceptor from the exhaust port andcommunicating with the interior volume of the vessel; supply linesconnecting a supply of at least one processing gas to the gasintroduction inlet; the gas introduction inlet being disposed parallelto the wafer supporting surface and generally centered on the axis andspaced from the wafer supporting surface to direct a flow of theprocessing gas from the gas introduction inlet into the processing spaceparallel to the axis, toward and perpendicular to the wafer supportingsurface of the susceptor; a wafer holder carried by the susceptor andoperative to hold the wafer to the supporting surface for processingwith a surface thereof centered thereon and facing the gas introductioninlet; and stationary annular opposed gas directing surfaces distributedwithin the volume uniformly around the axis so as to direct the gas in asmooth non-turbulent flow from the gas introduction inlet, across thewafer and past the susceptor to the exhaust port, the gas directingsurfaces forming at least one baffle, surrounding the axis and axiallypositioned between the wafer supporting surface of the susceptor and theexhaust port, so as to facilitate the exhausting of gas through theexhaust port with non-turbulent flow inside the processing space. 27.The apparatus of claim 26 wherein:the at least one baffle comprises aplurality of axially spaced baffles each defining an annular passagearound the axis; the passages have cross-sectional areas that decreasewith their proximity to the exhaust port; and the processing space isbounded by a vessel housing and the susceptor has an outer wall spacedfrom the housing to form a passage therebetween that has across-sectional area greater than those of the passages.
 28. Theapparatus of claim 26 wherein:the apparatus is a CVD reactor and theprocessing gas includes at least one reactant gas; and the susceptor hasan annular lip surrounding the wafer support surface, closely spacedtherefrom and flush with the surface of a wafer, when supported on thewafer support surface, to reduce turbulence and radial thermal gradientsin the wafer near the edge thereof.
 29. The apparatus of claim 26wherein:the gas introduction inlet is downwardly facing to direct a flowof processing gas downward into the processing space from above; and thewafer supporting surface is upwardly facing.
 30. An apparatus forprocessing semiconductor wafers comprising:a sealed vessel enclosing aninterior volume, the vessel having having an exhaust port connected toone end of the volume and having a vacuum pressure level maintainedtherein; a susceptor supported on an axis in a processing space in theinterior volume of the vessel, the susceptor having a wafer supportingsurface thereon oriented perpendicular to the axis; a gas introductioninlet disposed at one end of the volume opposite the susceptor from theexhaust port, the vessel and connectable to a supply of at least oneprocessing gas; the gas introduction inlet being disposed parallel tothe wafer supporting surface and generally centered on the axis andspaced from the wafer supporting surface for directing a flow of theprocessing gas from the gas introduction inlet into the processing spaceparallel to the axis, toward and perpendicular to the wafer supportingsurface of the susceptor; a wafer holder carried by the susceptor andeffective to hold a wafer to the supporting surface for processing witha surface thereof centered on the susceptor and facing the gasintroduction inlet; and gas directing surfaces distributed uniformlyaround the axis to direct gas in a smooth non-turbulent flow from thegas introducing inlet, across the wafer and past the susceptor to theexhaust port, the gas directing surfaces including a smoothly contouredexternally curved shaped outer wall on the susceptor to minimizingturbulence in the flow of gas in the processing space.
 31. An apparatusfor processing semiconductor wafers comprising:a sealed vessel enclosingan interior volume, the vessel having a vacuum exhaust port at one endof the volume through which the volume is maintainable at a vacuumpressure level; a susceptor supported on an axis in a processing spacein the interior volume of the vessel, the susceptor having a wafersupporting surface thereon oriented perpendicular to the axis; a gasintroduction inlet disposed at one end of the volume opposite thesusceptor from the exhaust port, connected to the interior volume of thevessel and connectable to a supply of at least one processing gas to thegas introduction means; the gas introduction inlet being disposedparallel to the wafer supporting surface and generally centered on theaxis and spaced from the wafer supporting surface for directing a flowof the processing gas from the introduction means into the processingspace parallel to the axis, toward and perpendicular to the wafersupporting surface of the susceptor; a wafer holder carried by thesusceptor and operable to hold a wafer to the supporting surface forprocessing with a surface thereof centered thereon and facing the gasintroduction inlet; and gas directing surfaces distributed uniformlyaround the axis for directing the gas in a smooth non-turbulent flowfrom the gas introduction inlet across the wafer and past the susceptorto the exhaust port, the gas directing surfaces including an annular lipon the susceptor surrounding the wafer supporting surface, the liphaving an inside opening which is close to, a circular outer edge of awafer supported on the wafer supporting surface, the lip having an outersurface positioned flush with the surface of the wafer, thereby reducingturbulence and thermal gradients in the wafer near the edge thereof inan outward direction perpendicular to the axis.
 32. The apparatus ofclaim 31 wherein:the lip has a smoothly contoured circular outer rimmeans for reducing turbulence in the processing space.
 33. The apparatusof claim 31 wherein:the annular lip is a first annular lip and theinside opening thereof is dimensioned so that the lip is close to acircular outer edge of a wafer of a first size, when supported on thewafer supporting surface, and the outer surface being flush with thesurface of the wafer of the first size for reducing turbulence andradial thermal gradients in the wafer near the edge thereof; theapparatus further comprises a second lip, annular in shape, having aninside opening dimensioned so that the lip is close to a circular outeredge of a wafer of a second size that differs from the first size sothat the susceptor accommodates wafers of different sizes, the secondlip having an outer surface flush with the surface of the wafer of thesecond size to reduce turbulence and radial thermal gradients in thewafer near the edge thereof; and at least said second lip beingremoveably and attachable to the susceptor.
 34. An apparatus forprocessing semiconductor wafers comprising:a sealed vessel having asealed housing enclosing an interior volume, the vessel having anexhaust port connected to one end of the volume for maintaining thevolume at a vacuum pressure level, the housing including a susceptormount; a susceptor supported on the susceptor mount in a processingspace in the interior volume of the vessel, the susceptor having a wafersupporting surface thereon and a hollow interior bounded by a susceptorwall having a highly reflective inside surface to reduce heat transferfrom the susceptor to the wafer supporting surface of the susceptor andto the susceptor mount; and a wafer heater effective for heating a waferheld on the supporting surface to a processing temperature.
 35. Anapparatus for processing semiconductor wafers comprising:a sealed vesselhaving a sealed housing enclosing an interior volume, the vessel havingan exhaust port connected to one end of the volume to maintain thevolume at a vacuum pressure level, the housing including a susceptormount; a susceptor supported on the susceptor mount in a processingspace in the interior volume of the vessel, the susceptor having a wafersupporting surface thereon and an interior bounded by a susceptor wallhaving an outside surface of sufficiently low reflectivity as toincrease radiation of heat away from the susceptor, thereby reducingheat transfer between the wafer supporting surface and the mount; and awafer heater operative to heat a wafer held on the supporting surface toa processing temperature.
 36. An apparatus for processing semiconductorwafers comprising:a sealed vessel having a sealed housing enclosing aninterior volume, the vessel having an exhaust port connected to one endof the volume, and a susceptor mount; a susceptor supported on thesusceptor mount in a processing space in the interior volume of thevessel, the susceptor having a wafer supporting surface thereon and ahollow interior bounded by a thin susceptor wall for reducing heattransfer between the wafer supporting surface and the susceptor mount;and a wafer heater operative to heat a wafer held on the supportingsurface to a processing temperature.
 37. An apparatus for processingsemiconductor wafers comprising:a sealed vessel having a sealed housingenclosing an interior volume, the vessel having an exhaust portconnected to one end of the volume and a a susceptor mount; a susceptorsupported on the susceptor mount in a processing space in the interiorvolume of the vessel, the susceptor having a wafer supporting surfacethereon and low thermal conductivity mounting material between thesusceptor and the mount for securing the susceptor to the mount and forproviding a thermal block to reduce heat transfer between the wafersupporting surface and the mount; and a heater operative to heat a waferheld on the supporting surface to a processing temperature.
 38. Anapparatus for processing semiconductor wafers comprising:a sealed vesselhaving a sealed housing enclosing an interior volume, the vessel havingan exhaust port connected to one end of the volume, the housingincluding a susceptor mount; a susceptor supported on the susceptormount in a processing space in the interior volume of the vessel, thesusceptor having a wafer supporting surface thereon and means includinga hollow interior bounded by a susceptor wall; the mount including firstmounting structure on the susceptor wall and second mounting structuresecured to the mount, the first and second mounting structure being incontact with each other, and the second mounting structure having areduced cross-sectional area presenting a thermal contact surface havingan area sufficiently small to minimize heat transfer at the interface ofthe first and second mounting structure; and means for heating a waferheld on the supporting surface to a processing temperature.